1. Field of the Invention
This invention relates to mechanisms providing limited relative rotation between a stationary element and a movable element and, more particularly, to flexural pivot devices having movable flexural members, such as flat springs, connecting the stationary element and the movable element for relative rotation.
2. Description of the Prior Art
In certain mechanisms, such as needle galvanometers, gyroscopes and pressure transmitters, where pivoting of some element through a small angle with very low and very reproducible resistance is required, flexural pivot devices have been used. Examples of flexural pivot devices are described in U.S. Pat. No. 3,277,555, issued to KUTASH, U.S. Pat. Nos. 3,073,584, 3,181,918 and 3,825,992, issued to TROEGER and U.S. Pat. No. 3,722,296, issued to HURLBURT et al.
Each of these flexural pivot devices uses flexural members which are crossed, flat springs made of steel, a material which is ultimately subject to fatigue due to elastic deformation. Accordingly, the allowable loading of flexural pivot devices using steel springs must be reduced approximately 50% to extend their useful life from 35,000 cycles to "indefinite". The useful life for any load is further reduced as the angular deflection, i.e., bending stress, is increased. More simply, where steel springs are used, the useful life decreases due to fatigue effects as the load and/or angular deflection is increased.
In contrast to steel, there is a host of alloys, called "shape memory" alloys, which do not exhibit fatigue because they deform plastically. When a shape memory alloy is annealed at a very high temperature (typically 550.degree. C. or above) in a given shape, the shape is fixed permanently, known as the "austenitic phase," unless it is annealed once again. If the shape memory alloy is cooled down below a certain temperature (typically 45.degree. C.-80.degree. C.), known as the "transition temperature," it becomes quite weak mechanically, known as the "martensitic phase," and can be deformed relatively easily. After deformation, if the temperature of the alloy is again raised above the transition temperature, the crystal structure of the alloy changes from the martensitic phase to the austenitic phase and the alloy recovers its originally fixed shape. Both the annealing temperature and the transition temperature depend on the particular shape memory alloy chosen.
An example of a shape memory alloy is Nitinol which is made substantially of 55% nickel and 45% titanium. Nitinol exhibits good strength and ductility and is characterized by an 8% shape recovery. Other shape memory alloys are, for example, 52.5% Au - 47.5% Cd; 80% In - 20% T1; and various combinations of Cu, Zn, Al and Ni.
Although shape memory alloys, such as Nitinol, have been used in certain products such as toys (see U.S. Pat. No. 4,244,140, issued to KIM), heavy machinery (see U.S. Pat. No. 4,010,455, issued to STANGE) and oil well sealing caps (see U.S. Pat. No. 4,424,865 issued to PAYTON, JR.), shape memory alloys have heretofore not been used in devices capable of effecting relative rotation, for example, flexural pivot devices.
Thus, although the prior art flexural pivot devices described above using steel springs have been reasonably effective, these prior art flexural pivot devices are not capable of an indefinite useful life over a wide range of loads and angles of deflection.